Acute glomerulonephritis (AGN) and acute rheumatic fever (ARF) are recognized as complications of streptococcal infections. They rarely occur in the same individual at the same time and different factors appear to be involved in their pathogenesis. Four epidemics of AGN have occurred in Trindidad during the past 13 years with unusually high endemic rates between the peaks. The most recent epidemic was in 1971. AGN usually is associated with skin infections in Trinidad. Although ARF also is common in the same population, it has not increased in incidence during the major epidemics of AGN and does not appear to be associated with the skin infections. Streptococcal strains and serum samples from patients and control subjects continue to be available to us. Thus, an unusual and valuable opportunity to compare many infections associated with AGN and ARF is available to us in Trinidad. Our broad objective is two-fold: First, to learn more about the pathogenesis of AGN and ARF by studying and comparing the streptococcal strains and the epidemiology of the streptococcal infections associated with AGN and ARF, respectively, and by examining several aspects of the host response in patients with these infections. Definition of nephritogenic and rheumatogenic factors would enable development of more effective treatment and prevention of these diseases. The second aspect of our objective is to increase our knowledge of the natural history of epidemic AGN and its relation to chronic disease by 10 and maybe 20 year follow-up of patients in Trinidad. A 10 year follow-up of patients ill in the 1965 epidemic is being planned for next year, and a similar study of patients with endemic AGN in 1966 for the following year.